[Orgmode] Refile cache expire

[Tom]

Caching of refile targets is great, however, I usually have my 
computer and emacs running for several days (I use hibernation),
so sometimes I need to clear the cache manually to have it rebuilt.

How about adding an option to set an expiration time for the refile
cache after which it is cleared automatically and rebuilt again?

For example, I could set the cache to expire after and hour (or a day,
etc.) and then it would be cleared and rebuilt automatically, but only
when the computer is idle for, say, 5 seconds. This way one could have
a pretty up-to-date refile cache all the time and the rebuilding
would happen when emacs is idle, so the use wouldn't even have to
notice the updating process.